Design of nanoscale molecular wire based on diphenylacetylene: Role of Linkage

The first proposal of using molecules for electronic applications was put forward by Aviram and Ratner in 1974 [1]. Since then various ideas to use molecules for electronic applications have been proposed. It is important to realize that the coexistence of molecules and metallic electrodes will probably form the basis of future molecular devises. Keeping this in mind, the simplest molecular electronic unit that can be envisioned is a two-terminal deviceconsisting of two metallic electrodes several nanometers apart, joined by a molecule anchored to them. This system is generically known as a molecular bridge or molecular junction and plays a central role in molecular electronics. The anchored molecule (molecular wire) is an organic molecule that forms a conducting bridge between electronic contacts. Construction of a molecular wire requires an elongated molecule consists of conjugatedmolecular units, which contains alternating single and double (or triplet) bonds through which electron can flow easily from one endto other. It is known that the delocalized π-conjugation plays animportant role in electron transportation.The higher thedelocalization, the easier transfer. May be we can say that theefficiency of molecular wires are understood by the differencebetween the energy of the highest occupied (HOMO) and the lowestunoccupied molecular orbital (LUMO) which is called LUMOHOMOgap (HLG). To achieve the conductance property atmolecular level a reduction in HLG is the prerequisite.It should be noted that any component in electronic devices must besubject to a considerable external electric field (EF). Under thisparticular condition, the molecular geometry as well as theelectronic structure, which plays a curial role in determining theconductance of molecular wire, is doubted to be the same as in the zero EF [2]. Therefore, a detailed study of EF effect on the electronic and geometric structure of molecular wire is highly desired for precisely understanding the molecular electrical properties. The purpose of this research is to simulate a molecular junction in order to study the interface between the molecule andthe electrode.